Light is one of the most important time cues synchronizing an organism's internal circadian timekeeping system (CTS) with the external environment. Desynchronization of the CTS may be responsible for many sleep and psychiatric disorders. Different colors of light have been shown to have different efficacy in treating seasonal affective disorder in humans. In mammals, light stimulates the retina transmitting temporal information to the suprachiasmatic nucleus (SCN), the neuroanatomical location of the biological clock. Only a few studies have tried to characterize the spectral and scotopic qualities of light entraining the cts in mammals. These studies have been performed on nocturnal rodents which have primarily rod photoreceptors in the retina. Primates have both rod and cone photoreceptors in the retina making it extremely difficult to extrapolate results obtained in rodents to human and nonhuman primates. It is hypothesized that the squirrel monkey may posses a multipart cts with differential sensitivity to light intensities and wavelengths. Our objective is to characterize the circadian responses of the rhythms of body temperature and activity in squirrel monkeys exposed to light-dark (ld) cycles of varying durations, intensities, and wavelengths. Adult male squirrel monkeys have been surgically implanted with biotelemetry transmitters to record body temperature and activity. The animals are unrestrained, have free access to food and water, and are isolated from extraneous environmental stimuli. The light wavelengths used range from blue to near infrared (400 - 1000 nm). Near infrared does not produce heat. The animals are first entrained to an ld cycle (e.G., Ld 12:12) of a given wavelength. The light intensity, at each wavelength, is reduced by 50% approximately every two weeks until the animal's cts starts to free-run, i.E., Is no longer entrained to the ambient ld cycle. Our results reveal that different l have different j thresholds for entrainment.